Progress during the past year includes: Huang et al., Stem Cells, 2015 Geminin is a dual-function protein unique to multicellular animals with roles in modulating gene expression and preventing DNA re-replication. Here, we show that geminin is essential at the beginning of mammalian development to prevent DNA re-replication in pluripotent cells, exemplified by embryonic stem cells, as they undergo self-renewal and differentiation. Embryonic stem cells, embryonic fibroblasts, and immortalized fibroblasts were characterized before and after geminin was depleted either by gene ablation or siRNA. Depletion of geminin under conditions that promote either self-renewal or differentiation rapidly induced DNA re-replication, followed by DNA damage, then a DNA damage response, and finally apoptosis. Once differentiation had occurred, geminin was no longer essential for viability, although it continued to contribute to preventing DNA re-replication induced DNA damage. No relationship was detected between expression of geminin and genes associated with either pluripotency or differentiation. Thus, the primary role of geminin at the beginning of mammalian development is to prevent DNA re-replication-dependent apoptosis, a role previously believed essential only in cancer cells. These results suggest that regulation of gene expression by geminin occurs only after pluripotent cells differentiate into cells in which geminin is not essential for viability. Adler-Wailes et al., submitted for publication Pluripotent stem cells, exemplified by embryonic stem cells (ESCs), are essential for mammalian development, but also a potential source of cancer stem cells. In vivo, ESCs give rise to all of the cell lineages. However, when placed at ectopic sites, ESCs produce germ cell neoplasia in the form of teratomas (benign) or teratocarcinomas (malignant). When cultured in vitro, ESCs require at least one geminin allele to prevent DNA re-replication dependent apoptosis during self renewal, a characteristic that diminishes after ESC differentiation in vitro. To determine whether or not this is also true within the animal, mice were inoculated subcutaneously with ESCs harboring conditional Gmnn alleles and a tamoxifen dependent Cre recombinase. Early intraperitoneal injection of tamoxifen limited tumor formation to those ESCs that escaped Gmnn ablation. Complete Gmnn ablation in established tumors eliminated the pluripotent stem cells, but it did not prevent proliferation of the differentiated Gmnn /- cells within the tumor, although they did not survive in vitro. Thus, geminin was essential for ESCs to differentiate into multiple cell types, but not for the viability of differentiated cells within a solid tissue. These results also provide proof of principle that selective inhibition of geminin function could convert malignant germ cell cancer into a benign tumor.